Conventionally, in the photolithography technique, an exposure device for transferring a fine circuit pattern onto a wafer and thereby producing an integrated circuit is widely utilized. With higher integration and higher functionalization of integrated circuits, the microsizing of the integrated circuit is advancing. The exposure device is hence required to form a circuit pattern with high resolution on a wafer surface in a deep focal depth, and shortening of the wavelength of the exposure light source is being advanced.
The lithography technique using a EUV light, typically a light having a wavelength of 13 nm, as an exposure light source is considered to be applicable over generations that a line width of a circuit pattern is 32 nm, and is attracting attention. The principle of image formation in the EUV lithography (hereinafter abbreviated as “EUVL”) is identical with that of the conventional lithography from the viewpoint that a mask pattern is transferred using a projection optical system. However, since there is no material capable of transmitting light therethrough in the EUV light energy region, a refractive optical system cannot be used. Accordingly, the optical systems are all reflecting optical systems.
The optical system member of an exposure device for EUVL (optical member for EUVL) is such as a photomask and a mirror, and is basically configured with (1) a substrate, (2) a reflective multilayer formed on the substrate, and (3) an absorber layer formed on the reflective multilayer. For the reflective multilayer, it is investigated to form an Mo/Si reflective multilayer in which an Mo layer and an Si layer are alternately laminated, and for the absorber layer, Ta and Cr are investigated as a forming material. For the substrate used for producing an optical member for EUVL (optical substrate for EUVL), a material having a low coefficient of thermal expansion is required so as not to generate a strain even under irradiation with EUV light, and a glass and the like having a low coefficient of thermal expansion is investigated.
It is known that the coefficient of thermal expansion of the glass material is decreased by incorporating a metal dopant. In particular, a silica glass containing TiO2 as a metal dopant, that is, a TiO2—SiO2 glass body is known as an extremely low thermal expansion material having a lower coefficient of thermal expansion than that of a silica glass. Additionally, because the coefficient of thermal expansion can be controlled by TiO2 content in the silica glass, a zero expansion glass having a coefficient of thermal expansion close to zero can be obtained. Therefore, the TiO2—SiO2 glass has a possibility as an optical substrate for EUVL.
However, one of drawbacks of the TiO2—SiO2 glass body is that the glass body has striae (see Patent Document 1). The striae are inhomogeneity on composition (composition distribution) adversely affecting light transmission of an optical substrate for EUVL prepared using the glass body. The striae can be measured with a microprobe which measures composition variation correlating with variation of a coefficient of thermal expansion of several ppb/° C.
When the glass body is used in an optical substrate for EUVL, the optical surface of the optical substrate for EUVL is required to be finished such that the surface roughness (PV value: difference between the highest point (Peak) and the lowest point (Valley) in design configuration of a processed surface) is very small. However, it was found that when the surface roughness (PV value) is finished to a level of several nanometers, the striae strongly affect in some cases. The “optical surface of an optical substrate for EUVL” used herein means a film-formed surface on which a reflective multilayer film is formed, in preparing an optical member for EUVL such as a photomask or a mirror using the optical substrate for EUVL. The shape of the optical surface varies depending on the purpose of use of the optical substrate for EUVL. In the case of an optical substrate for EUVL used in the production of a photomask, the optical surface is generally a flat surface. On the other hand, in the case of an optical substrate for EUVL used in the production of a mirror, the optical surface is often a curved surface.
For this reason, to use the TiO2—SiO2 glass body in an optical substrate for EUVL, the striae are required to be reduced.
Patent Document 1 discloses a method for manufacturing an element for an extreme ultraviolet lithography (optical substrate for EUVL) comprising: a step of providing a silicon-containing feedstock and a titanium-containing feedstock; a step of delivering the silicon-containing feedstock and titanium-containing feedstock to a conversion site; a step of converting the silicon-containing feedstock and titanium-containing feedstock into titania-containing silica soot; a step of consolidating the titania-containing silica soot into an inclusion-free, homogeneous titanium-containing silica glass preform; and a step of finishing the titanium-containing glass preform into an element for an extreme ultraviolet lithography (optical substrate for EUVL) in which a stress caused by striae is less than 0.05 MPa.
In the method described in Patent Document 1, the conversion site has a furnace having exhaust vents and the striae level is maintained by controlling exhaust vent flow during the production process. Or, the striae level is modified by adjusting the distance between the preform and the burner. Or, the striae level is reduced by depositing the soot in a cup mounted on a vibration table and increasing the rotation rate of the vibration table.
However, to implement those methods, great modifications are required to be added to the existing facilities, which is not preferred. Furthermore, the implementation of those methods results in decrease in productivity of an optical substrate for EUVL, which is also not preferred. Additionally, the implementation of those methods, because bubbles and foreign matters are easily incorporated in a glass, is not preferred.
Patent Document 2 describes that striae of a TiO2—SiO2 glass body are reduced by heat-treating the TiO2—SiO2 glass body at a temperature higher than 1,600° C., specifically, by heat-treating in a temperature range of from 1,600 to 1,700° C. for from 48 to 1,600 hours.
According to Patent Document 2, the striae of the TiO2—SiO2 glass body can be reduced. However, since the heat treatment is conducted at extremely high temperature, this gives rise to the problems of foaming and subliming in the TiO2—SiO2 glass body, which is not preferred. Furthermore, for the heat treatment at high temperature, a carbon-made mold material must be used and a carbon furnace must be used. As a result, the peripheral portion is reduced to blacken and crystallize. This gives rise to the problems that such a glass body cannot be used as a product and a peripheral heterogeneous layer is increased.